As a general description, printers function by depositing ink from a printhead horizontally across a print medium (e.g., a page of paper). The page is advanced to its next position, and ink is applied horizontally to another part of the page. There may also be instances in which ink is applied repeatedly to the same part of a page. In any case, it is important that the printhead be accurately located relative to the page, in order to deliver ink to precise locations on the page. The inability to accurately deliver ink can cause discernible defects on the printed page.
Optical navigation technology (ONT) provides a mechanism that can be implemented in printer systems to determine relative positioning of the printhead and print medium. Applying ONT to printers involves very rapidly capturing multiple successive images of the surface of the print medium. The images can be compared, differences can be identified and analyzed, and the differences can be used to calculate the change in position from a previously calculated position.
Some ONT-based systems that can be used in printers rely on the features and patterns of the print medium to determine displacement of the print medium. These types of systems can be based on the fact that blank white paper, for example, is not uniform when observed at high, magnification. By taking advantage of the non-uniformity of the paper, patterns in the paper can be detected and used to identify a location on the page. While innovative, these types of systems can give inconsistent results.
Other ONT-based systems that can be used in printers use distinct surface characteristics or patterns, such as alpha-numeric characters appearing on the print medium, to more precisely navigate the print medium. A first image of the print medium can be obtained, followed a short time later by the acquisition of a second image different from the first. Although different, the two images have some area of commonality. The difference between the two images indicates how far the printhead has traveled, allowing for its location to be accurately determined.
The images of the print medium can be obtained using an imager or photo array. More precise navigation can be obtained by increasing the size of the photo array. With a larger photo array, the signal-to-noise ratio is increased, allowing the relative position of the printhead and the print medium to be more accurately deduced.
However, larger photo arrays are problematic because they are costly. Also, fabrication of larger photo arrays means larger dies and fewer dies per wafer. Larger dies are more susceptible to spot defects on the wafer, so the fraction of defective photo arrays per wafer will increase.
Thus, it is desirable to improve navigation accuracy without increasing the size of the photo array.